1. Field of the Invention
The present invention generally relates to magnetic recording media and methods of producing the same and magnetic storage apparatuses, and more particularly to a magnetic recording medium which is suited for high-density magnetic recording, a method of producing such a magnetic recording medium, and a magnetic storage apparatus which uses such a magnetic recording medium.
2. Description of the Related Art
Due to the progress made in the information processing technology, there are demands to improve the performance, such as increased storage capacity and high transfer rates, of magnetic disk apparatuses which are often used as external storage units of computers. In order to improve the performance of the magnetic disk apparatus, it is necessary to improve the performance of the magnetic recording medium which records information, and it is essential to reduce the media noise and to increase the coercivity of the magnetic recording medium.
Conventionally, various magnetic recording media having a magnetic layer with a multi-layer structure have been proposed. For example, a Japanese Laid-Open Patent Application No.5-114128 proposes a magnetic recording medium having a magnetic layer with a multi-layer structure, and this multi-layer structure is made up of a lower magnetic layer made of a CoCrTa-based alloy and an upper magnetic layer made of a CoCrPt-based alloy. In this proposed magnetic recording medium, the coercivity of the magnetic layer having the multi-layer structure does not become higher than the coercivity of a magnetic layer having a single-layer structure made up of a similar alloy. Hence, compared to the magnetic layer having the single-layer structure, the magnetic layer having the multi-layer structure has a reduced media noise, but the recording and reproducing resolution is not improved and it is difficult to obtain a high signal-to-noise (S/N) ratio at a high recording density.
On the other hand, a Japanese Laid-Open Patent Application No.6-243454 proposes a magnetic recording medium having a magnetic layer in which a normally-magnetic region with low Co concentration and a high Cr concentration is disposed between two regions with a high Co concentration, wherein a plurality of such regions are provided in a direction in which a thickness of the magnetic recording medium is taken. According to this proposed magnetic recording medium, the media noise is reduced because the decreased mutual interaction of the regions in the magnetic layer. However, since the magnetic layer basically has a multi-layer structure in which the magnetic layers have the same composition, the coercivity does not become higher than the coercivity of the magnetic layer having the single-layer structure made up of a similar alloy. For this reason, compared to the magnetic layer having the single-layer structure, the recording and reproducing resolution is not improved and it is difficult to obtain a high S/N ratio at a high recording density.
Furthermore, a Japanese Laid-Open Patent Application No.10-320740 proposes a magnetic recording medium having a multi-layer structure using alloys such as a CoCrPt-based alloy, CoCrPtTa-based alloy and a CoCrPtNi-based alloy, wherein lower magnetic layers have a higher Pr concentration than upper magnetic layers and the upper magnetic layers have higher Cr concentrations than the lower magnetic layers. In this proposed magnetic recording medium, the coercivity is adjusted by the Pt-content of the upper magnetic layers, and thus, the coercivity is greatly affected by the magnetic characteristic of the upper magnetic layers. In addition, because the upper magnetic layers have a larger Cr-content than the lower magnetic layers, the recording and reproducing resolution may deteriorate in a high-frequency region of the recording and reproducing characteristic.
As described above, the previously proposed magnetic recording media employ the multi-layer structure for the magnetic layer, mainly for the purposes of reducing the media noise. In addition, in the magnetic recording media having the magnetic layer with the multi-layer structure, the thickness of each of the magnetic layers in the multi-layer structure becomes small compared to the thickness of the magnetic layer having the single-layer structure.
FIG. 1 is a diagram for explaining a decrease in coercivity with decreasing thickness of a CoCr-based magnetic layer having a single-layer structure. In FIG. 1, the ordinate indicates the coercivity Hc (¼π*kA/m) and the abscissa indicates the thickness (nm) of the magnetic layer having the single-layer structure. As shown in FIG. 1, the coercivity decreases as the thickness of the magnetic layer having the single-layer structure decreases. Because of this tendency for the coercivity to decrease with decreasing magnetic layer thickness, it was considered extremely difficult to obtain a high coercivity with the magnetic layer having the multi-layer structure. Consequently, when the magnetic layer having the multi-layer structure is employed, the recording and reproducing resolution is low, and even if the media noise is slightly reduced, it was difficult to obtain a high S/N ratio for the high-density recording.